1. Field of the Invention
This invention relates to magnetic bubble memories and more particularly, to such memories adapted for fault handling.
2. Description of the Prior Art
Magnetic bubble memories are now well known in the art. A particularly familiar organization for bubble memories is disclosed in P. I. Bonyhard, U.S. Pat. No. 3,618,054 issued Nov. 2, 1971. That patent describes the "major-minor" organization for a field-access bubble memory. The term "field-access" refers to a memory wherein bubble propagation paths are defined by a pattern of elements typically of high permeability permalloy. The elements are designed to respond to a magnetic field reorienting in the plane of bubble movement to move domains to a detector. Since bubble movement occurs in response to periodic changes in a magnetic field and since sequential access of data represented by bubble patterns occurs in response to those changes, the arrangement is referred to as a "field-access" arrangement.
In major-minor organizations, the bubble paths are defined as closed loops, operative in parallel, for recirculating bubble patterns. An accessing path to which information is moved from the minor loops is operative synchronously to move bubble patterns to a detector. The accessing path is disposed close to one end of the minor loops and is called the major path. This major path, or a similar major path close to the opposite ends of the minor loops, is adapted for a write operation in which information, generated therein at a generator, is advanced into positions for movement to the minor loops.
It has been found that during manufacture, magnetic bubble memories are characterized by a yield which is substantially limited by the fact that a small number of minor loops exhibit diminished operating margins and/or defects, such as an impaired element, which preclude or limit operation. Of course, the presence of normally unused minor loops which could be substituted for such defective loops would improve yields substantially. But, some means for ensuring proper organization of the data stream from an impaired set of minor loops plus a replacement loop must be provided because the replacement loop cannot be located physically on the chip for proper placement of the data bit from that loop.
In prior art bubble fault-handling schemes, the proper placement of data from a replacement loop with respect to the data stream required significant circuitry external to the bubble memory. Thus, for example, prior art schemes for fault-handling typically employ a read-only memory (ROM) in which data recording the positions of faulty loops are stored. The output of the ROM is used to first strip the data stream of the bits to be stored in the auxiliary memory and second, to provide the needed control to reconstruct the data stream.